In the automotive industry structural panels are used in a wide variety of applications where high strength and lightweight are required. Moulded reinforced panels are particularly for use in an automotive vehicle as for instance a parcel shelf, ceiling cover, engine bay panels or load floor as well as for panels used at the outside of a car like an under engine shields or outer wheel arch liner. Additional acoustic properties for the attenuation of noise can be a requirement, in particularly the sound absorption factor of the material. For instance composite panels, eventually with a honeycomb core, are used in trim parts, sunroof panels, hard tops, parcel shelves, spare wheel covers and luggage floor assemblies. Depending on the material chosen, they can also be used as under floor, engine or engine-bay cover. Fiber reinforced composites are used as the main material or as a skin layer for these products, sometimes combined with additional layers for specific purposes.
Composite materials (or composites for short) are engineered materials made from two or more constituent materials with significantly different physical or chemical properties, which remain separate and distinct on a microscopic level within the finished structure.
Composites are made up of individual materials referred to as constituent materials. There are two categories of constituent materials: Matrix and reinforcement. At least one portion of each type is required. The matrix material surrounds and supports the reinforcement materials by maintaining their relative positions. The reinforcements impart their special mechanical and physical properties to enhance the matrix properties. A synergism produces material properties unavailable from the individual constituent materials. Engineered composite materials must be formed to shape. The matrix material can be introduced to the reinforcement before or after the reinforcement material is placed into the mould cavity or onto the mould surface. The matrix material experiences a change in physical state, for instance for thermoplastic material a melting event, after which the part shape is essentially set. Depending upon the nature of the matrix material, this change in physical state can occur in various ways such as chemical polymerisation (duroplast) or solidification from the molten state (thermoplastic).
Most commercially produced composites use a polymer matrix material often called a resin solution. There are many different polymers available depending upon the starting raw ingredients. There are several broad categories, each with numerous variations. The most common are known as polyester, vinyl ester, epoxy, phenolic, polyimide, polyamide, polypropylene, PEEK, and others. The reinforcement materials are often fibres, but also commonly ground minerals. Composite material can be made by using a layer or mat of fibrous material at least partially consisting of reinforcing fibers like glass fibers, and a binder material, either in form of a powder, a liquid solution or as binder fibers. The materials are mixed and cured, normally by heat moulding the material in a moulding press producing directly the wanted product form.
US20050214465 discloses a process for producing a composite using polyamide as a matrix whereby the reinforcing materials are impregnated with a lactam melt activated for anionic polymerisation and afterwards heated. Another known process is the pultrusion process. The material produced can be granulated and later on used in injection moulding or extrusion methods.
Another technique used is mixing the reinforcement fibres with the thermoplastic melt. Also here mostly followed by injection moulding eventually followed by press moulding to obtain the desired form of the products.
The use of a thermoplastic melt or impregnation with a melt renders the product obtained compact and non-porous, as the melt will fill up the spaces between the reinforcing material and close all existing pores.
U.S. Pat. No. 7,132,025 discloses a process using thermoplastic fibres as matrix material. These fibres are first blended with the reinforcing fibres and than dry-laid to give a blended web. The web is than consolidated with needling, heated and compacted to give the final product. The web is heated to a temperature above the softening point of the thermoplastic fibres using a conventional oven or by IR radiation and directly compressed to provide a compressed and partially consolidated thermo-formable semi finished product.
US20050140059 discloses a process of producing moulded parts made of fibres whereby the fibres are first heated between plates and then subjected to compression moulding, using additionally air suction to obtain a better-shaped product. The fibres used are bicomponent fibres as binder fibres and other fibres like reprocessed cotton and polypropylene as the bulk fibres. Although the use of high-pressure steam or fluid air as alternatives for the heating of the material before compression moulding are mentioned in the introduction, the actual disclosed process only uses heated plates to obtain 200° C. for one minute to heat and consolidate the fibrous material. The use of steam is not disclosed in combination with the used materials and the disclosed process.
WO2004098879 discloses a method of producing a composite material of a mixture of thermoplastic fibres and reinforcement fibres using a needled nonwoven web as the starting material. This web is combined with dual foils with a high melting and a lower melting thermoplastic material. The layered stack is than heated, using either IR-waves or hot air, up to such a temperature, that the thermoplastic fibres and the low melting thermoplastic material of the foil are heated above their melting temperature for a short time, long enough to enable a softening. Directly afterwards the layered material is pressed, for instance using rollers. The patent discloses as an example a combination of Polyamide-6 as the binder fibre and glass- and PET fibres as the reinforcement fibres.
Also WO2007000225 discloses a method of producing a stiff part using a combination of low and high melting fibres, whereby the fibre web is heated above the melting temperature of the low melting fibres. The application further discloses the use of glass fibres or polyester fibres as high melting fibres and polypropylene or polyester as low melting fibres in a core material. This core material is layered between 2 outer thermoplastic foil layers. During the heating step the inner core material is expanding because of inner pressure in the fibres of the core, giving a lofting effect to the overall material. The final product contains partly highly compressed areas and partly this lofted areas. In praxis this is done with a combination of polypropylene and glass fibres and is called soft lofting.
A disadvantage of the state of the art is the high temperature needed to obtain the final composite. The heating temperature to be achieved is dependent on the matrix polymer. To form the composite, the matrix and reinforment fibres are heated using a dry heating method like hot air, contact heating or infrared heating. The product is normally heated above the true melting point of the matrix polymer to compensate for the temperature loss for instance from the heating device to the moulding device. Heating of a polymer above the melting point accelerates degradation.
Using a contact heater has the additional disadvantage that the product has to be compressed to obtain a good transfer of heat throughout the thickness of the product. Hot air is normally used at a temperature above the melting temperature of the binder polymer thus the polymer gets heat damaged, while the use of infrared heating is only feasible for thin materials. In thicker materials the amount of energy needed to heat the inner core is damaging for the outer surface polymers. This method is normally used only for a thickness up till 4-5 mm.
Another disadvantage is the fact that most thermoplastic polymers used as matrix fibres and as reinforcement fibres have their melting temperature close to each other for instance the melting temperature of poly ethylene terepthalate (PET) is in the range of 230-260° C., for polypropylene between 140-170° C., for Polyamide-6 between 170-225° C. and for Polyamide-6.6 between 220-260° C. Using matrix fibres and reinforment fibres both being thermoplastic polymers, for instance PA6.6 as matrix and PET as reinforcement, having to heat them above the melting temperature of the matrix fibres will also cause the reinforcement fibres to start melting or softening. This will lead to a collapse of the structure, forming a very compact composite.
The felts are widely used particularly in automotive industry for their thermal and acoustic insulation properties. The trend is towards recyclable materials; therefore thermoplastic binders have taken a significant share in the last years. Fibers made of high performance polymers such as polyesters, polyamide are highly interesting due to their mechanical and heat resistance properties. But the necessary binding agent form the limitation to their utilization in moulded 3D parts.
The binding agents used so far always have a lower melting point than the reinforcement fibres, rendering in relatively weak performance behaviour to the moulded fibre web and limiting its utilization to tempered areas in the vehicle. None of these types of moulded fibre webs is suitable for the high temperature exposure of the engine bay or compartment, particularly of the engine contact areas. Some of these binders are modified polymers (CO-PET as an example) having pour behaviours due to their modified structure particularly sensitive to hydrolysis phenomena.
The processes for moulding such felts as known in the state of the art are a “cold” moulding process where the felt is pre-heated by various means, and then transferred to a cold mould in which it is compressed in order to get the part shape or a “hot” moulding processes, where the felt is introduced in a closed mould, in which a heat transfer media, like air, is introduced for bringing the binding agent to its melting point, and then released. The part is then cooled down, inside the tool or outside, with or without cooling assistance. (See for instance EP1656243, EP1414440, and EP590112)